1. Field of the Invention
This invention relates to a light control apparatus.
2. Description of the Related Art
In recent years, a digital information recording system using the principle of hologram has been known as a large-capacity recording method (Patent Document 1, for instance).
FIG. 5 illustrates an example of a hologram recording apparatus. A hologram recording apparatus 100 principally includes a laser light source 102, a beam splitter 104, a beam expander 106, a spatial light modulator SLM 108, a hologram pattern writing means 110, a Fourier transform lens 112, a recording medium 114, a mirror 116 and a rotating mirror 118. Here, a transmissive display apparatus is used as a spatial light modulator SLM 108.
In the hologram recording apparatus 100, laser light emitted from the laser light source 102 is split into two beams of light by the beam splitter 104. One of the beams of light undergoes an enlargement of beam diameter by the beam expander 106 and is irradiated to the spatial light modulator SLM 108 as parallel light. The hologram pattern writing means 110 transmits a hologram pattern to the spatial light modulator SLM 108 as an electrical signal. The spatial light modulator SLM 108 forms a hologram pattern on a plane based on the electrical signal received. The light irradiated to the spatial light modulator SLM 108 is light-modulated by passing through the spatial light modulator SLM 108 and becomes a signal light containing a hologram pattern. This signal light is subjected to a Fourier transform as it passes through the Fourier transform lens 112 and is focused into the recording medium 114. On the other hand, the other of the beams of light, which has been split by the beam splitter 104, is led as reference light to the inside of the recording medium 114 through the mirror 116 and the rotating mirror 118. Inside the recording medium 114, the light paths of the signal light containing the hologram pattern and the reference light cross each other to form an optical interference pattern. The whole interference pattern is now recorded as a change in refractive index (refractive index grating) in the recording medium 114.
In a hologram recording apparatus 100, an image of one frame is recorded in this manner in a recording medium 114. Upon completion of the recording of the image of one frame, the image of a second frame is recorded using the same procedure, by turning the rotating mirror 118 by a predetermined amount and at the same time moving the position thereof parallely by a predefined amount, thereby changing the angle of incidence of reference light in relation to the recording medium 114. By repeating a processing like this, an angular-multiplexing recording is carried out.
The material that can be used for the spatial light modulator SLM of a hologram recording apparatus is, for instance, lead lanthanum zirconate titanate (hereinafter referred to as PLZT) or the like which has an electro-optical effect. PLZT is a transparent ceramic which has a composition of (Pb1-yLay) (Zr1-xTix)O3. An electro-optical effect is a phenomenon in which an electric field applied to a substance causes polarization in the substance with a resulting change in its refractive index. Using the electro-optical effect, the phase of light can be switched by turning the applied voltage on and off. Accordingly, a light modulating material with an electro-optical effect can be applied to the optical shutter of a spatial light modulator SLM or the like.
In applications to these optical shutter and other devices, bulk PLZT has been widely used conventionally (Patent Document 2). However, it is difficult for the optical shutters using bulk PLZT to meet the demands for miniaturization or higher integration or the demands for lowered operating voltage or lowered cost. Besides, the bulk method involves a process of treating at high temperatures of 1000° C. or above after the mixing of material metal oxides, and hence, if applied to a device forming process, it may place many constraints on the selection of materials, the structure of the device and the like.
Under these circumstances, there have been attempts at applying a thin film of PLZT formed on a base material, instead of bulk PLZT, to light control devices. Described in Patent Document 3 is a display apparatus for which a PLZT film is formed on a glass or other transparent substrate and comb-shaped electrodes are placed thereon. This display apparatus has such a structure that a polarizing plate is provided on each of both faces of a display substrate with a PLZT film formed thereon. Here, the electrode terminal of each pixel is connected with an external drive circuit, so that desired pixels are driven and thus a desired display can be produced by transmission light from a light source provided on one face of the display substrate.    [Patent Document 1]
Japanese Patent Application Laid-Open No. 2002-297008.    [Patent Document 2]
Japanese Patent Application Laid-Open No. Hei05-257103.    [Patent Document 3]
Japanese Patent Application Laid-Open No. Hei07-146657.
(First Problem to be Solved)
However, if a light modulating film such as a PLZT film or the like as described above is to be put to practical use as an optical shutter device or the like, it will be necessary to form a drive circuit for controlling the on and off of the voltage applied to the light modulating film, together with the light modulating film, on a substrate. In so doing, however, the structure as described in aforementioned Patent Document 3 has the problem that the area where the drive circuit is formed cannot be used as a display area and hence an effective display area cannot be secured sufficiently.
Also, when visible light is used as the irradiation light, a transmissive display apparatus as described above had the problem that the drive circuit cannot be formed on a silicon or like substrate which is opaque to visible light.
Moreover, with a display apparatus as described in Patent Document 3, which uses polarizing plates, a loss of light due to the polarizing plates occurs.
(Second Problem to be Solved)
A discussion will be made of a method for applying an electric field to a light control device using a thin-film PLZT as described in Patent Document 3. When an electrode using Au, IrO2, Al, or the like as material is formed on the surface of PLZT, the electrode part does not allow the penetration of light, so that the opening ratio and light utilization efficiency will drop inevitably. Thereupon, the inventors have attempted an improvement in which the electrodes formed on PLZT are formed by the use of transparent electrodes in order to further improve the light utilization efficiency.
ITO (Indium Tin Oxide) is known as a representative material for a transparent electrode. The inventors have formed electrodes on a PLZT film using this ITO as the electrode material and measured the electrical characteristics thereof. FIG. 8 is a graph showing relations between applied electric field and polarization when an opaque electrode is formed using IrO2 and when a transparent electrode is formed using ITO. For both the materials, sputtering was used to form the electrodes.
From FIG. 8, it is apparent that the amount of polarization in response to an application of the same electric field decreases markedly when the electrode is formed of ITO in comparison to when the electrode is formed of Ir/IrO2.
Also, the relative permittivity was measured, and as a result it became clear that the relative permittivity, which was ∈=1270 when the electrode was formed of Ir/IrO2, dropped to ∈=820 when the electrode was formed of ITO.
(Third Problem to be Solved)
The inventors have investigated a light modulating apparatus having a Fabry-Perot resonator structure, in which a PLZT film is used as the light modulating film as an example of applying a thin-film PLZT to a light control device and a reflection layer is provided on both sides of the light modulating film. This light modulating apparatus, which is provided with a light modulating film and two reflection layers provided in such a way as to hold the light modulating film in between, controls light by changing the refractive index by the electric field applied to the light modulating film and thereby shifting the resonance wavelength of the light modulating apparatus.
In Fabry-Perot resonator, the resonance wavelength λm is given byλm=(2nt cos θ)/m  (Equation 1)where m is a degree, n is a refractive index within the resonator, t is the length of the resonator, and θ is an incident angle of laser light within the resonator. Here, the film thickness of the light modulating film corresponds to the length t of the resonator.
Since the resonance wavelength λm is proportional to the resonator length t, variation in film thickness of the light modulating film results in variation in resonance wavelength. That is, an extremely high accuracy is required of the film thickness t of a light modulating film if the reproducibility of the resonance wavelength λm is to be achieved.
Here, when an electric field is applied in the thickness direction of light modulating film, it is necessary to make the film thickness as thin as about 1 μm because of its relationship with voltage if a sufficient electric field is to be obtained. However, it is difficult to form the film thickness of a PLZT film, which is so thin, with a high accuracy of about 1%.